Sound film reproducing system



Patented Aug. 12, 1941 SOUND FILM REPRODUCING SYSTEM Y Arthur C'. Blaney, Los Angeles, Calif., assgnor to Radio Corporation of America, a corporation of Delaware Application April 22, 1940, Serial No. 330,813

8 Claims. (Cl. 179-1003) 'I'his invention relates to motion picture apparatus and particularly to a sound lm reproducing system.

In the art of sound recording and reproduction, it is well known that there are many different types of recording and reproducing systems which are distinguishable from one another by the form of the sound nlm record produced and reproduced. For instance, there are ilm sound records known as unilateral and bilateral variable area, standard and push-pull class A variable area, and push-pull class B variable area, and

substantially similar types of records of the variable density form. a

It is also known that not only is a certain type of recording system required -to produce a certain type of record, but usually each type of record requires a diierent type of reproducing system. That is, a sound record of the standard variable area or standard Variable density type is reproducible with a single photoelectric cell employed as the pickup element in the reproducing system, while the push-pull types of sound records require a divided, twin or two photoelectric cells connected in a push-pull relationship.

In the past, standard types of recording and reproducing systems have been employed both at the studios for the original recording, for editing playbacks and in the theatres. The present-trend of sound recording, because of .the better quality obtainable, is toward the use of push-pull-sound records for the original recording and then, by rerecording, to convert the records into the standard type for reproduction in the theatres. If the iinal theatre record were in push-pull form, it would require the conversion of all theatre installations to the two photocell type of reproduction circuits and this is not feasible at the present time. Even in the studios, where the sound records are reproduced many times during editing, the reproducing systems require Vmodiiication to the push-pull type which has caused many studios to hesitate to use the higher quality push-pull track.

The usual form of editing device used by the cutters or editors in the studios exhibits the picture and reproduces the concomitant sound from individual lms. These devices have a magnifying glass or small viewing screen for the picture lm which is separate from the sound reproducing portion of the device, the latter being of the standard single 'photoelectric cell type. The sound reproducing portion of the unit does not reproduce with high qualitybut such quality is not required for editing purposes.

The present invention is directed to a reproducing system which by a slight adjustment may be readily converted from the basic standard type employing a single photoelectric cell to one which willsatisfactorily reproduce push-pull sound records of all types. To reproduce the class A pushpull type of record, it is only necessary to block orf one-half of the record, since each trace of the record contains the complete modulations of the signal. Thus by insertion of a mask to cover half the record, the standard single cell reproducer is usable for reproduction.

It is well known, however, that the push-pull class B type of recording has advantages over the standard and push-pull class A type of recording, the B type of record thus being employed more and more for original recording. For the reproduction of push-pull class B records, both traces must be scanned and combined in the proper sequence to reproduce the entire signal, each trace having the modulations occupying mutually exclusive areas transversely of the sound track area. Thus, each trace must ordinarily be detected by a separate photosensitive device connected in a push-pull circuit, and one trace of a class B pushpull record, therefore, will not reproduce theforiginal signal. With the present invention the pushpull class B record may be satisfactorily reproduced with a single photoelectric cell in the standard circuit now in general use.

The primary object of the invention, therefore, is to facilitate the reproduction of push-pull sound records.

Another object of the invention is to reproduce a class B push-pull sound record with a standard single photoelectric cell reproducing system.

A further object of the invention is to modify present standard ilm sound record reproducing systems to reproduce class B sound records.

A further object of the invention is to reproduce a push-pull class B sound record by modulating a single photoelectric cell with both traces of the sound record.

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity in the claims appended herewith, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawing forming a part thereof, in which Figure 1 is a diagrammatic drawing embodying the invention;

Figure 2 is a fragmentary vieW of a push-pull class A sound lm record;

Figure 3 is a fragmentary view of a push-pull class B sound film record; and

Figure 4 is a graph showing the operation of the reproducing circuit in accordance with the invention.

Referring now to Fig. 1, light from a source 5 is collected by a lens 6 and projected upon a slit mask 8 having a slit 9 therein. The light passing the slit 9 is collected by objective lenses l0 andV projected upon a iilm Il having a sound track l2 thereon. The emerging light from the nlm impinges upon a photoelectric cell I4, the output of which is amplified by an amplier I5 and reproduced in a loudspeaker I6. The system so described is a basic type of standard sound reproducing system now in general use in the exhibition of sound motion pictures. There has been provided, however, in the present system a rotatable adjustment for the slit mask 8 by threading, its outer circumference and gearingV it to a gear 20 manuallyrotatable by a iinger knob 2|. Although this particular manner of adjusting the slit mask 8 has been shown, it is to be realized that other means of adjustment may be employed.

In the operating of the above system, the lenses I8 focus the slit 9 upon a sound track portion I2 of the lm l'l and, ordinarily, the direction of movement of the lm is normal to the longest dimension of the light image falling on the film. Even though the slit image is not accurately adjusted so that its length is exactly normal to the direction of movement of the film, reproduction from normal or push-pull class A records is. not affected deleteriously. This is because the usual form of sound record is of the single or double Vtrace type wherein each trace contains the complete modulations of thesignal. This is illustrated by referring to Fig. 2, wherein one form of push-pull class A variable area track is shown. It will be noted that each trace is completeV and continuous during` the time oi the signal and that each track is of the same form, except for a 180 out-of-phase relationship.

Ordinarily, in the reproduction of such a record, the light passing through each trace from a slit normal to the direction of movement of the film impinges upon an individual photoelectric cell, which,Y being connected in a pushpull relationship, will reproduce the signal at the highest efficiency. However, outside' oi' an amplitude decrease, either trace is reproducible by itself with! a single photoelectric cell circuit i by simply blocking orf the light passing through one of the traces.

In the reproduction of a class B push-pull type of record where one trace represents the positive component and the other trace, the negative component of a signal, as shown in Fig. 3*, the modification for a class A track will not suiiice for reproduction inasmuch as each trace carries substantially one-half of the signal modulations, and, therefore, the reproduction of only one of the traces is unintelligible. However, it has been discovered that such a track may be reproduced with sufficient intelligibility for editing and review room purposes by orienting the slit 8 in the reproducing system so that it is at an angle in the neighborhood or 30 to the usual slit position. Through many experiments with various angles, it has been found that certain angles give optimum intelligibility in reproduction in accordance with the type of selection, but for the general use of both dialogue and music, the angle of 30 has been found desirable.

To explain why this invention permits the reproduction of such a record with a single photoelectric cell circuit, reference is made to Fig. 4, wherein the solid line a. represents a sinusoidal wave which, we will assume, has been recorded in the form of a push-pull class B record. If only one trace oi the B record were reproduced, the result in the reproducing system would be only the upper or positive portions of the curve a. If both traces oi the class B record were'reproduced by projecting both traces on a single photoelectric cell, the reproduction would then be of the double frequency type wherein not only would the positive portions above the central line o be reproduced, but also the lower negative portions would appear as shown by the dotted. lines b. This type of scanning, of course, will not reproduce the record.

However, by rotating or orienting the reproducing slit to an angle of approximately 30, as shown in Fig. 3 by the dotted lines, and projecting the light from both traces to a single photoelectric cell, the resulting sound would correspond substantially'to a wave as shown by the dot-and-dash lines c. 1t will be observed that a certain amplitude distortion is present, although the frequency is the same as that of the original wave. The nal reproduced arnplitude, of course, may be easily controlled by amplification. Although the illustration in Fig. 4 is of a simple harmonic sound, even complicated musical selections are satisfactorily reproduced in the same manner, while dialogue is completely intelligible and thus is satisfactory for editing and review room purposes.

By embodying means in a reproducing system or by modifying existing types of reproducing systems so that the reproducing slit may be rotated or oriented to the proper degree, the conversion of a standard type of reproducing system to the push-pull class B type of system is accomplished quickly and simply. The rotation of the slit must be such that the light reaching the photoele'ctric cell does not substantially decrease between adjacent negative and positive modulations in the respective traces in accordance with the particular 30 angle used. That is, a 30 angle in the other direction is also satisfactory. Thus, for studio use wherein various types of sound records are encountered, the operator need only inspect the track, and if it be of the standard type, the slit will be positioned normal to the direction of movement of the iilm; if it be of the push-pull class A type, the slit remains normal to the direction of lm but a mask may be inserted for eliminating one-half of the light through the track; and if it be of the push-pull class B type, the slit is rotated to an angle of approximately 30 With respect to the usual position of the slit. If the record is a 'class B recording of a particular selection, the operator may adjust his slit during reproduction to the optimum angleY for that particular selection or form of recording. l

What I claim as my invention is:

1. The method of reproducing a push-pull sound record comprising illuminating said record with a narrow beam of light positioned at an angle of substantially 30 to a line normal to the direction of movement of said record, and impressing the light emerging from said record on a photoeleetric cell.

2. The method in accordance with claim 1 in which said push-pull record is of the class B type having substantially parallel traces of discontinuous modulations, said beam of light always contacting a modulation in each of said traces.

3. The method of reproducing a sound record having substantially parallel traces of discontinuous modulations, the modulations lying in mutually exclusive areas transversely of said sound record area, comprising simultaneously "scanning the adjacent modulations in each of said tra'ces With a beam of light and impressing a photoelectric cell with the light emerging from said record.

4. The combination of means for reproducing a push-pull sound record having a plurality of traces, said means comprising means for producing a beam of light, means for projecting said light to said traces at mutually exclusive positions transversely of said sound record area, a photosensitive device, and means for impressing the light emerging from both oi said traces on said device.

5. A combination in accordance with claim 4 in which said light projecting means includes means for forming a narrow light beam, and

means for adjusting said last-mentioned means, the adjustment of said beam being at an angle of substantially 30 to a line normal to the direction of movement of said record.

6. A combination in accordance with claim 4 in Which said light projecting means includes an adjustable slit mask, the slit thereof having an adjusted position at substantially 30 to a line normal to the direction of movement oi said record.

'7. A push-pull sound film reproducer for a sound record having a trace for the positive components and a separate trace for the negative components of a signal comprising a means for projecting a beam of light to adjacent positive and negative components -in respective traces, a single photosensitive device, and means for simultaneously impressing the light from said traces on said device.

8. A method of reproducing a sound record having a trace for the positive components and a separate trace for the negative components of a sign-al comprising simultaneously scanning said adjacent positive and negative components in respective traces, and detecting the light varia- -tions resulting from said scanning operation.

ARTHUR C. BLANEY. 

